In a spark ignition engine, an air-fuel mixture in a combustion chamber is ignited by a spark plug, and conventionally, the spark plug is ideally attached to the center of the combustion chamber. When ignition is performed in the center, the resulting flame propagates concentrically from the center toward the periphery.
Having approached the periphery, however, the flame is cooled by the wall surface of the combustion chamber and may be extinguished as a result. If the flame is extinguished, unburned gas is discharged as is through an exhaust valve. When the excess air ratio is approximately 1.7, for example, the fuel concentration of the air-fuel mixture in a cylinder is approximately 90,000 ppm, and when the flame is extinguished on the peripheral portion, the amount of hydrocarbon discharged in an unburned state during a low load operation may exceed 7,000 ppm. Accordingly, no less than 8% of the fuel is wastefully discharged. Moreover, the amount of HC and CO discharged in the exhaust gas increases, thereby worsening the exhaust performance.
Hence, the applicant has investigated a method of disposing a plurality of spark gaps around a cylinder opening portion and performing ignition from the periphery of the combustion chamber, where the flame is more likely to be extinguished. According to this method, the flame propagates from the plurality of peripheral spark gaps toward the center, and thus the air-fuel mixture in the combustion chamber can be burned rapidly and flame extinction on the peripheral portion can be suppressed. Moreover, by reducing the combustion time, the ignition timing, which is conventionally before top dead center, can be retarded, whereby loss that is generated when the rising piston is pushed back down by the explosion can be suppressed. As a result, the output and fuel efficiency of the engine can be improved.
As a result of a search conducted by the applicant into techniques for performing ignition using a plurality of spark gaps disposed on the periphery of a combustion chamber, JP57-185689A and JP58-175279A were found.